I've worked as a sysadmin for some years and what I keep coming back to is that users like Microsoft Outlook and want to use its Exchange features. I have tried my fair share of commercial alternatives but usually there is either a fundamental feature missing or there are stability issues.

In short I am looking for a Microsoft Exchange Alternative with the following features:

Authentication through SQL or LDAP

Has a solid, comfortable web interface for the users when they are off-site

Supports replication and load balancing (if one fails, the second one should be already running)

Outlook client support (or a really good alternative client)

Resource booking (meeting rooms, projectors, company jet, etc)

Calendar (shared/private) and Email (if that wasn't obvious)

(Optional) A cross-platform client for us *nix users.

(Optional) Corporate support contracts available

(Optional) An open-source software is a plus

Please keep your answers as detailed as possible to determine that you've successfully deployed the software and it fulfills the needs. If I wanted a list of claimed alternatives , I would simply Google it.

I've personally tried Binary Server, Novell Groupwise, homegrown Postfix/Cyrus stuff and in the end the 'real thing' because those users just love Exchange.

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Excellent question and I'm also interested in the answers. Additionally, I'm on the lookout for a client that is a viable alternative to Outlook. Cross-platform would be ideal but at the very least it must run on Windows. Evolution is the nearest I've found in functionality but that won't run on Windows.
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John GardeniersJul 4 '09 at 14:09

12 Answers
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Zimbra is an excellent opensource, linux based alternative to Exchange. It combines Apache Tomcat, Postfix, MySQL, OpenLDAP and Lucene in a single, well defined package. It offers:

LDAP Authentication

Calendaring, resource booking and free/busy info

Ability to connect using outlook, and its own Zimbra client

Excellent web interface

Allows multi server setup and replication

It is available for free, with the option of paying for a supported version. I have used Zimbra for a number of organisations who did not, or could not pay for Exchange, and they have all been very happy with it.

According to the Zimbra web page you get the Outlook connector if you buy the enterprise version.
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AndrioidJul 4 '09 at 13:21

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I've most often used the network edition, which does have the connector, but yes it does appear that you only get the outlook connector with the network edition, but its still substantially cheaper than Exchange. Alternatively you can use the Zimbra desktop client, or use POP or IMAP to connect with outlook.
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SamJul 4 '09 at 13:26

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I use Outlook with Zimbra where I work. It has always worked fine for me.
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ScottJul 4 '09 at 16:05

@scott: do you use it with just imap for email or do you also use it for calendering as well?
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chrisJul 11 '09 at 22:56

At my last job I migrated us off of Intermedia's Exchange service to Kerio Mail Server. It has great webmail (much easier and lighter than OWA), LDAP, ActiveSync and support for NotifyLink/Sync. It looks like Exchange to anyone using Outlook or Entourage. GAL, resource booking, calendars, built-in spam and AV.

Stores everything in flat RFC822 files so no broke Exchange store, has a great management interface and stellar support.

It really does most anything Exchange does, but is more light weight, doesn't require hughe hardware or licensing costs. I loved it while I was on it.

Another vote for KMS here. We've been running our business on it for over a year now and it's far simpler to administer than any Exchange setup I've worked with in the past. We're using it because we're a purely Mac and Linux shop, but the business types still wanted to have Outlook (or Entourage in our case).
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Kamil KisielJul 5 '09 at 16:51

I've deployed it in the past as a decent replacement for Exchange. The Outlook plugin can lag a bit a bit behind the current version of outlook - took 6 months for a plugin for outlook 2007, for example.

Last time I checked the prices were ok, but not fantastically favourable as opposed to Exchange.

The Google Apps option is a great option to consider, especially for small to mid-size business. I've used it for several small domains, as well as assisted a medium-sized University in transitioning their mail systems, all with positive results. Just be careful with any custom requirements. If you need to archive email, or run concurrent mail systems, the complexity can increase dramatically. Just make sure to perform due diligence for your requirements before making the switch.
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SteveMJan 3 '10 at 13:56

Yes, I'm a little late for the party and not a little surprised that OX hasn't been mentioned. I'm pretty sure it meets and/or exceeds the OP's requirements. And for another option, keep your ear to the rails to see what Cisco does with PostPath which was a promising Exchange alternative.

If pushed we’d have to say that the Z-twins (Zimbra and Zarafa) offer, arguably, the best level of Exchange/Outlook compatibility, with little to choose between them, and Scalix comes a close third. But then buyers are unlikely to base their decisions solely on the Outlook experience, with lots of other features and omissions to consider which could swing the balance in other directions.

I installed the network version of Zimbra for a company last summer. It meets everyone of your needs plus support palm, blackberry and windows mobile over the air sync. It was very slick and the Outlook connector worked very well for not only mail but Calendaring and resource scheduling as well. It has a great web front end and *nix users can use Thunderbird & Firebird to access e-mail, contact list and calendaring or use the Zimbra full client which is basically the web client running on a local light weight web server.

It offers a MAPI provider that allows Outlook to connect almost exactly as it would if you were using Exchange. I can confirm from personal experience that it works well, runs on Linux, and very few of my users know the difference.